1. Field of the Invention
The present invention relates to the treatment of underground well formations, and more particularly, but not by way of limitation, to an improved method of placing a foamed treating fluid comprising a foamed acid or foamed fluid containing proppant material in the upper portion of substantially vertical fractures present in the formation.
2. Brief Description of the Prior Art
Many various methods have been developed and used for increasing and stimulating the production of oil, gas, and the like from an underground formation penetrated by a well bore. In treating such underground formations, particularly those formed of relatively impermeable and fracturable materials, it has become common practice to induce fractures in the most productive zone of the formation. The fractures then are extended laterally outward from the well bore to facilitate the flow of desired fluids from the formation into the well bore. In most formations, induced fractures are vertically oriented, that is, the fracture or fractures produced lie in a substantially vertical place paralleling the axis of the well bore. The induction of such fractures generally is accomplished by pumping a fluid known in the art as a "fracturing fluid" through the well bore into the formation. The fracturing fluid is utilized to exert hydraulic pressure on the formation at a location in the most productive and desired portion of the formation. The fluid pressure creates one or more fractures in the formation which then are extended outwardly from the well bore by continued pumping of the fracturing fluid at high pressure into the formation.
After the fractures have been created, the faces of the fractures can be etched with acid so that when the formation is returned to production and the fractures close, high capacity flow channels remain in the faces of the fractures through which desired fluids contained in the formation flow to the well bore. The acid can be introduced into the formation as the fractures are being produced and extended into the formation. This technique of fracturing and acidizing a formation to create flow channels is known in the art as "fracturing acidizing".
In an alternative method of producing high capacity flow channels in the formation, proppant material is introduced into the formation. After the fractures have been created and extended in the formation, a propping agent such as sand, glass beads, sintered bauxite or the like can be placed within the fractures to maintain the fractures open after fluid pressure on the formation is reduced and the fractures close.
In fracture acidizing as well as other fracture treatment procedures heretofore used, less than optimum results often have been achieved due to the treating fluid gravitating into the bottom portion of the fracture or fractures leaving the upper portion thereof untreated. This is particularly undesirable in those cases in which the fracture extends into a water zone which, if treated, will produce excessive amounts of water and thereby limit or prevent desired fluid production. Typically, the fracturing fluids and acidizing fluids of these known treatment procedures were aqueous or hydrocarbon solutions.
In U.S. Pat. No. 3,954,142, assigned to the same assignee as the present invention, a process for introducing a treating fluid into a selected region within a fracture is disclosed. In the practice of that invention, a preflush fluid first is introduced into the fracture. A treating fluid having a density sufficiently different from the density of the fluid then is introduced into the fracture whereby the treating fluid is segregated and caused to flow into a desired region within the fracture. To introduce a treating fluid into the upper portion of a substantially vertical fracture in accordance with that invention, for example to avoid a water zone, a high density preflush is pumped ahead of a less dense treating fluid so that the treating fluid will override the higher density preflush. In the event a high density, high viscosity preflush is used to fracture the formation, a low density, low viscosity fluid can be pumped into the formation to create a flow channel over the top of the preflush. A low viscosity fluid must be used because high viscosity fluids tend to move through a fracture in a formation as a mass with less tendency to be affected by differing densities than low viscosity fluids. The low viscosity fluids readily override and underride fluids of differing density.
The use of such fluids is undesirable when liquid-sensitive or permeable formations are to be treated. Large amounts of liquid may cause swelling of clays in the formation or reduce the permeability of the formation to produced fluids. Alternatively, fluid loss additives that often are required in permeable formations may impair the formation conductivity.
To overcome these problems, treatments employing foamed treatment fluids have been developed. Typically, foams contain only 20 to 40 percent liquid. Foamed fluids exhibit low fluid loss characteristics without the use of fluid loss additives. Further, foamed fluids can support proppant material and distribute the proppant more uniformly throughout a fracture than many other fluids.
When an attempt is made to employ foamed treating fluids in the manner disclosed in U.S. Pat. No. 3,954,142 the system is incompatible. The viscosities of foamed fluids are relatively high in comparison to nonfoamed fluids. To introduce a foamed treating fluid into a fracture and cause it to override a foamed preflush, the foamed treating fluid should be less dense and have a lower viscosity than the preflush. If a lower viscosity foam is used, generally such a foam is made by reducing the liquid to gas ratio of the foam; however, this generally results in an increase in the density of the foamed fluid. If a lower density foam is required, generally such a foamed fluid is produced by increasing the liquid to gas ratio of the foam; however, this generally results in an increase in the viscosity of the foam. Thus, both of these systems are incompatible with the fluid characteristics necessary to create the flow channel over the foamed preflush.